The present invention is directed to a process for forming images, and more specifically, a process for forming secure images. Secure images are generally useful for applications such as passport photographs, identification badges, banknote paper, and the like. A secure image is formed by generating an image and transferring it to paper so that the image cannot be removed by mechanical or chemical means. Such an image is resistant to tampering and also prevents removal of the image and substitution of another image in its place, since any attempt at removal of the original image damages the paper. In one embodiment, the present invention is directed to a process for forming secure images which comprises electrostatically charging an imaging member; imagewise exposing the charged member, thereby forming a latent image on the member; developing the latent image with a liquid developer comprising a liquid medium, a charge control additive, and toner particles comprising a colorant and a polymeric material; allowing the developed image to dry on the imaging member; contacting the portion of the imaging member with the dry developed image with a substantially transparent sheet having an adhesive material on the surface thereof in contact with the imaging member, thereby transferring the developed image from the imaging member to the substantially transparent sheet; contacting the adhesive surface of the substantially transparent sheet with the developed image with a paper sheet having a polymeric coating on the surface that is in contact with the substantially transparent sheet; and applying heat and pressure to the substantially transparent sheet and the paper sheet at a temperature and pressure sufficient to affix the image permanently to the paper. The resulting document is a paper sheet covered with the transparent sheet, with the developer material that forms the image being situated between the paper sheet and the transparent sheet. The image is "secure" in that the transparent sheet bearing the image cannot be removed from the paper without irreparably damaging the paper.
Processes for transferring a developed image by applying adhesive material to the receiver sheet are known. For example, U.S. Pat. No. 2,297,691 discloses a process for transferring an image generated by electrophotographic means and developed with a dry powder developer to a receiver sheet to the surface of which has been applied an adhesive material such as water, other liquids, wax, paraffin, or other soft or sticky substances. In addition, U.S. Pat. No. 3,130,064 discloses a process for permanently affixing developed electrophotographic images to a support material such as a record card which entails treating the record card or other image support material with a coating of a thermoplastic organic resin compatible with the toner material, followed by application of heat or radiant energy. U.S. Pat. Nos. 2,221,776 and 2,357,809 also disclose transfer of an electrophotographic image to an adhesive substrate.
Additionally, U.S. Pat. No. 3,275,436 discloses a process for forming image reproductions wherein an adhesively tacky support base surface bearing a resist image is placed in contact against a second support base containing a releasable uniform surface film separable selectively by area subjected to adhesive attraction. The two support bases are then separated from each other, and the film from the second support base is released to the first support base in the surface areas devoid of the resist image.
Further, U.S. Pat. No. 4,064,285 discloses a process in which a toner image pattern is formed on a transfer member which is overcoated with a polymeric material. The polymeric material assists in the permanent adherence of the toner image to cloth or other substrate materials under heat and pressure. U.S. Pat. No. 4,066,802 discloses a process in which a toner image pattern is formed on a transfer member which has been overcoated with an abhesive material. A polymeric sheet is interposed between the toner image and a cloth or other image receiving medium. The polymeric sheet assists in the permanent adherence of the toner imaging pattern to the cloth material or other medium when the composite is subjected to heat and pressure.
In addition, U.S. Pat. No. 4,812,383, the disclosure of which is totally incorporated herein by reference, discloses a process for forming permanent electrophotographic images that comprises generating, in an electrophotographic imaging apparatus, an electrostatic latent image; developing the image with a liquid developer comprising a colorant, a solvent, and a polymeric material having adhesive properties when wetted with the solvent; transferring the image to a substrate having a coating comprising a polymeric material having adhesive properties when wetted with the liquid developer solvent; and permitting the image to dry on the substrate. The polymeric coating on the substrate preferably is of the same composition as the polymeric material in the developer, and may be a vinyl toluene acrylic terpolymer such as Pliolite.RTM.OMS.
Although the prior art processes are believed to be suitable for their intended purposes, a need remains for processes for forming secure images. A need continues to exist for processes wherein a secure image is formed and transferred to paper and cannot be removed without damaging the paper. In addition, a need exists for processes for forming secure images that are resistant to tampering. There is also a need for processes for forming secure images suitable for applications such as passport photographs, identification badges, and banknote paper.